Variable electrical resistances and potentiometers



June 32, 1956 P. x. Fox ZIFSQAM VARIABLE ELECTRICAL RESISTANCES AND POTENTIOMETERS Filed Oct. 11, 1954 2,750,48i Patented June 12, 1956 VARIABLE ELECTRICAL RESISTANCES AND POTENTIOMETERS Paul X. Fox, Horsforth, near Leeds, England Application October 11, 1954, Serial No. 461,591

10 Claims. (Cl. 201-453) This invention relates to variable electrical resistances and potentiometers of the kind provided with means for correcting the law by angular progression or regression of the contact making member on the winding relative to the angle of rotation of the driving shaft. As heretofore proposed, such a correction has been applied for the purpose of securing a linearity within a very small tolerance, but the increments of correction which can be obtained with the correction devices previously proposed have been small. The chief object of the present invention is, therefore, to provide an improved form of variable electrical resistance or potentiometer in which any desired correction of the law can be obtained, the device being particularly applicable to the production of a sine, cosine or other non-linear law.

The variable electrical resistance or potentiometer according to the present invention is characterised by the provision of a cam track by means of which up and down movement is imparted to at least one leg carried offcentre by a centrally pivoted contact slipper, together with means for keeping said leg in contact with the cam track, the arrangement being such that the controlled pivotal movement of the slipper, due to engagement of the leg with the cam track, alters the contact point of the slipper with the winding in addition to its normal adjustment due to the spindle drive, whereby the point of contact with the winding gains or loses in relation to the normal linear increment of movement.

Conveniently, the slipper is in the form of a segment of a circle which is less than a semicircle, the slipper being mounted on a resilient arm which takes up the vertical up-and-down movement of the pivot of the slipper as the point of contact of the slipper is advanced or retarded in relation to its central position.

Many other novel and advantageous features, both of arrangement and construction, are provided by the present invention, these being more fully described hereinafter and claimed in the appended claims.

In order that this invention may be the more clearly understood and readily carried into effect, reference may be made to the accompanying drawings, which illustrate by Way of example one convenient embodiment of the present invention in the form of a four-contact potentiometer, and in which:

Figure 1 is an under-plan view of the potentiometer,

Figure 2 is a front elevation of the same, showing the contacts in one position,

Figure 3 is a front elevation of a potentiometer, showing the contacts in a different position,

Figures 4 and 5 are, respectively, front elevation and plan view of a modified method of mounting and controlling a contact slipper according to the present invention, and

Figures 6A, B and C are diagrammatic views illustrating the manner in which the correction is obtained.

Referring now to the accompanying drawings, the cam track 10 is formed on the edge of an annular member 11 surrounding the potentiometer winding 12 and each of the four contact slippers 13 shown is centrally pivoted at one end of a spring arm 14 which is adapted to be directly driven by the driving spindle 15 of the potentiomete'r. In the embodiment shown in Figures 1 to 3 of the drawings, two legs 16 are provided outstanding from the slipper 13 fore and aft of the pivot 17, said legs having turned-up feet 18, or rollers engaging with the cam track iii). In the embodiment of the present invention illustrated in Figures 4 and 5 of the accompanying drawings only a single leg 16 is provided for each contact slipper, said leg being off-centre from the pivot 17, and a spring 16a is provided outstanding from a suitable part of the moving member bearing on the slipper 13 above the single leg 16 in order to keep its foot or roller 13 in contact with the cam track 1 0. Which of these two embodiments is to be preferred will depend entirely upon the law of compensation to be applied and hence on the shape of the cam track 10. In general it may be stated, however, that the modification of Figures 4 and 5 is more generally applicable than the embodiment shown in Figures 1 to 3.

The slipper 13, as will be appreciated from Figures 2 and 3, is in the form of a segment of a circle which is less than a semicircle, but the slipper may take any other arcuate shape provided it is adapted to roll over the potentiometer winding 12 in a manner which will be described more fully hereinafter.

it will be seen that as the driving spindle 15 is turned the slippers 13, whether they be controlled in the manner shown in Figures 1 to 3, or in the manner shown in Figures 4 and 5, will have two movements applied to them, one being the normal linear increment of movement imparted by the driving sprindle l5 and the other a rocking movement about the pivot 17 which will depend upon the shape of the cam track. This will be appreciated from Figures 6A, B and C which show in an exaggerated manner the manner in which the correction is obtained.

In Figure 6B the two legs 16 of the embodiment of Figures 1 to 3 lie at points of the cam track which are level with one another whilst in the case of the embodiment of Figures 4 and S the single leg is such that the top of the slipper is level, and so, in either case, the point of contact X of the slipper 13 with the winding 12 lies directly under the pivot 17. In Figure 6A, with the embodiment of Figures 1 to 3, the right-hand leg 16 is engaging with the cam track at a point which is lower than in the case of Figure 6B and, correspondingly, for the embodiment ofFigures 4 and 5. Hence the point of contact X of the slipper 13 with the Winding 12 is displaced to the right. In Figure 6C the reverse movement has taken place and the point of contact X on the slipper 13 on the winding 12 has been displaced to the left. Therefore, by suitably shaping the cam track 10 the point of contact X of the slipper 13 with the winding 12 can be caused to gain or lose in relation to its normal linear increment of movement to give any desired law.

The spring arms 14 carrying the contact slippers 13 enable the pivots 17 of the slippers 3 to rise and fall relative to the winding 12 to compensate for the shape of the slipper, the vertical upward and downward movement of the pivots 17 being obvious from Figures 6A, B and C.

Many modifications may be made in the embodiments above described without departing from the scope of the present invention. For example, although a multiple contact potentiometer has been illustrated, it will be obvious that the invention is equally applicable to a single contact potentiometer, and to a potentiometer having fewer or more contacts than the four illustrated. The contact 13 whilst preferably in the shape of a segment of a circle can be made of any other shape on its undersurface provided it is capable of rolling along the winding under the influence of the cam track and, in some cases, the under-surface of the slipper may be gapped, or cut away, at one or more points to give, in effect, a step on the output voltage from the resistance. Instead of being arranged outside the winding 12 of the potentiometer the member 11 carrying the cam track may equally well be arranged inside the same. Again, two or more cam tracks may be provided operating the same or different slippers, as desired, to enable different cornpensations to be obtained at the various contact slippers. The two or more cam tracks may also be made to move by a suitable drive independent of the winding and the spindle and slipper assembly, and this movement may be obtained by means of a cam and/or gears. This will enable one to set up, or adjust, one output law for one slipper and another output law for another slipper. The cam track may also be made in the form of two or more segments each adjustable with respect to the others whereby any necessary adjustments in the phasing of the contacts of the multiple contact device can be obtained. Again, the cam track may, if desired, have two faces, an upper face and a lower face so as to produce differential angles of rock on the slippers 13. Furthermore, the winding 12 need not extend in the form of a complete circle, but may be in the form of an arc of less than 360, and further compensation may be obtained by shaping of the winding, by winding sections on the same with different gauges of wire, and the winding may also be tapped and have sections shorted out for special purposes.

With the correction devices according to the present invention one is no longer limited to small increments of correction as with the devices heretofore proposed, but one can obtain quite a large degree of correction and one can also ensure that the potentiometer will follow even the most complicated law desired, particularly with the modified forms of correction described above.

One of the factors controlling the degree of roll of the slippers, and hence the degree of correction applied, is the length of the contact face of the slipper, and by suitably co-relating the shape of the cam track to the length of the contact face of the slipper, the potentiometer may be made to follow any desired law or, in fact, to have any regular or irregular variation for equal increments of movement of the driving spindle.

The cam track may be profile moulded or profile blanked and may, in some cases, be blanked out of sheet which is subsequently formed circular.

It will be appreciated that although in the embodiments described above the contact assembly has been rotated whilst the resistance winding and cam track are held stationary, the converse arrangement may be employed, if desired, the resistance winding and the cam track being rotated, whilst the contact assembly is held stationary. In some cases, also, means may be provided for causing relative rotary movement between the potentiometer and the cam track.

I claim:

1. A variable electrical resistance or potentiometer, characterised by the provision of a cam track by means of which up and down movement is imparted to at least one leg carried off-centre by a centrally pivoted contact slipper, together with means for keeping said leg in contact with the cam track, the arrangement being such that the controlled pivotal movement of the slipper due to engagement of the leg with the cam track alters the contact point of the slipper with the winding, in addition to its normal adjustment due to the spindle drive, whereby the point of contact of the slipper with the winding gains or loses in relation to the normal linear increment of movement.

2. A variable electrical resistance or potentiometer according to claim 1, characterised in that the slipper is in the form of a segment of a circle which is less than a semicircle, the slipper being mounted on a resilient arm which takes up the vertical up-and-down movement of the the pivot of the slipper as the point of contact of the slipper is advanced or retarded in relation to its central position.

3. A variable electrical resistance or potentiometer according to claim 1, wherein said slipper is provided with a single leg mounted off-centre and adapted to engage with the cam track, spring means being provided resiliently engaging said slipper to keep said leg in engagement with the cam track.

4. A variable electrical resistance or potentiometer according to claim 1, wherein said slipper is provided with two legs mounted fore and aft of the central pivot, said legs being adapted to engage with said cam track.

5. A variable electrical resistance or potentiometer according to claim 1, wherein the under-surface of the slipper is arcuate and has a gap formed therein for providing an abrupt change in the position of contact between said slipper and said winding.

6. A variable electrical resistance or potentiometer according to claim 1, wherein said cam track is carried by an annular member arranged at one side of said winding.

7. A variable electrical resistance or potentiometer according to claim 1, further comprising means for adjusting the cam track mechanically.

8. A variable electrical resistance or potentiometer occording to claim 1, which the cam track is in the form of a plurality of segments each adjustable with respect to one another, whereby any necessary adjustment in the phasing of the contacts can be obtained.

9. A variable electrical resistance or potentiometer according to claim 1, wherein the winding of the potentiometer is given a law other than a linear law by shaping the former and/or winding portions of the same with wire of different gauges.

10. A variable electrical resistance or potentiometer according to claim 1, wherein the winding is tapped, means being provided whereby sections can be shorted out.

References Cited in the file of this patent UNITED STATES PATENTS 

